This invention relates to a heterojunction bipolar transistor and a method of manufacturing the transistor.
Recently, an interest has been shown in heterojunction bipolar transistors (hereinafter referred to it as HBTs) for application to a power amplifier, a digital IC and the like as a ultra-high-speed, super high-frequency device. According to "Suppression of Emitter Size Effect on Current Gain in AlGaAs/GaAs HBTs" by O. Nakajima et al., Jpn.J.Appl.Phys., Vol.24, No.10, 1985, pp. 1368-1369, it is known that an AlGaAs/GaAs HBT, having a graded-bandgap base construction, has high current gain, compared with an HBT having a uniform bandgap base construction.
FIG. 6 is a section showing a construction of a simple mesa npn HBT having a conventional graded-bandgap base construction according to O. NakaJima et al. In the figure, the reference numeral 20 indicates a semi-insulating GaAs substrate, 21 indicates an n.sup.+ -GaAs collector contact layer (sub-collector layer), 22 indicates an n-GaAs collector layer, 23 indicates a compositional grading p.sup.+ -AlGaAs base layer, 24 indicates an n-AlGaAs emitter layer, 25 indicates an n.sup.+ -GaAs emitter cap layer, 26 indicates an emitter electrode, 27 indicates a base electrode, and 28 indicates a collector electrode.
A fundamental operation of the HBT with the above construction is briefly explained. A part of the electrons injected from the emitter layer 24 to the base layer 23 are recombined with the holes in the base layer 23 as a base current, and the other part of the electrons reach the collector layer 22 as a collector current. The transistor is operated by changing the collector current by controlling the base current. Since the emitter layer 24 is composed of AlGaAs, which has a large bandgap, a reverse injection of holes from the base layer 23 to the emitter layer 24 is prevented, which results in a high emitter injection efficiency and a large current gain. The high current gain is maintained even with the base layer 23 whose concentration of p-type impurity is high, which leads to a lower base resistance. Further, since the material of the collector layer 22 and the like is GaAs, which is excellent in electron transfer characteristics, a high-speed operation such as a reduced collector transit time is contemplated. Since a threshold value almost depends on a difference of bandgap between the base and the emitter, its stability is ensured, compared with an FET. With high transconductance and comparatively small characteristic deviation due to micro-fabrication, it is useful for integration.
Moreover, the HBT in FIG. 6 has the graded-bandgap base by giving a compositional grading to the base layer 23. The compositional grading of the AlGaAs base layer 23 is shown in FIG. 7. As is seen from FIG. 7, in the HBT, a composition of AlAs of the base layer 23 is continuously increased from an interface of the base layer 23 with the collector layer 22, to an interface thereof with the emitter layer 24, so as to continuously increase the bandgap of AlGaAs composing the base layer 23 from the collector/base interface to the emitter/base interface. In detail, the AlGaAs base layer 23 has 100 nm thickness and 0 AlAs composition (i.e. GaAs) at the collector/base interface and 0.1 AlAs composition (i.e. Al.sub.0.1 Ga.sub.0.9 As) at the emitter/base interface. With the graded-bandgap base construction, a built-in internal field whose intensity is about 12 kV/cm is caused in the base layer 23. The built-in internal field accelerates electrons so as to enhance the electron speed in the base layer 23, thus, improving the current gain, compared with the HBT with uniformed bandgap base construction. Wherein, as shown in FIG. 7, a concentration of a p-type impurity in the base layer 23 is uniform.
The built-in internal field intensity of the GaAs/AlGaAs HBT, with the graded-bandgap base construction, can be increased up to about 20-30 kV/cm at which intensity, transition is caused between the .pi. and L valleys. However, it is hard to further increase the built-in internal field intensity in the base layer 23 when AlGaAs is used as the material of the base layer 23. When the AlAs composition at the emitter/base interface is increased more than 0.2 in order to increase the grading of the bandgap of the AlGaAs base layer 23, an energy barrier of valence band between the emitter and base layers is decreased. As a result, it is apt to cause the reverse injection of holes from the base layer 23 to the emitter layer 24, and decrease the emitter injection efficiency. Consequently, the AlAs composition in the base layer 23 on the emitter side should not exceed 0.2. Therefore, the increase of the built-in internal field intensity in the AlGaAs base layer 23 with the graded-bandgap base construction is restricted.
In addition, compared with an HBT with a GaAs base layer, the AlGaAs base layer 23 has a higher base resistance to cause degradation of high-frequency characteristics. Beryllium (Be) is typically used as a p-type impurity in the AlGaAs base layer 23. With heavy doping for decreasing the base resistance, problems of variation per hour of collector current at high-current-density operation due to diffusion of Be, current gain decrease due to surface recombination and the like arise. It is also known that segregation of Be is liable to be caused in AlGaAs.